ST_CollectionExtract extracts highest coordinate dimension by default

[pramsey]

Add in a one-parameter mode where

ST_COLLECTIONEXTRACT('GEOMETRYCOLLECTION(LINESTRING, POINT))')

returns a multi-linestring. Basically the component of highest coordinate dimension is returned when the function is called without a specific return type.

[Paul Ramsey <pramsey@…>]

In 6812b00/git:

Add ST_CollectionExtract(geometry)
with a behaviour of extracting
the primitive geometry (point/line/poly) with the highest coordinate
dimension. So a collection of lines and polys will result in a
multipoly output. A collection of lines and points will result in a
multiline output.
Closes #4784